Laboratory investigation on gas permeability of compacted GMZ bentonite under a coupled hydraulic-mechanical effect

During the long-term evolution process of the geological repository, the gas permeability evolution of the buffer material is critical to its sealing performance evaluation. The purpose of this study is to investigate the gas permeability evolution of unsaturated GMZ bentonite under the coupled effect of stress-saturation gradient-gas pressure. First, the samples were held at different relative humidities and frees-swelling conditions to simulate the in-situ saturation gradient. The results show that the water absorption (or loss) of bentonite can affect its water content, dry density and porosity, further affecting its gas permeability characteristics. The evolution of the gas permeability is influenced by the coupling of these factors, which is between 10⁻¹⁹ and 10⁻¹⁵ m². During the first cycle of low confining pressure, the results show that the values of the gas permeability are very close, despite the large difference in water contents. Further analysis found that in addition to the water content, the change in pore structure (pore volume) also affects the gas permeability. The effects of the two basically offset each other, and as a result, the change in gas permeability is not very clear. After undergoing a cyclic loading and unloading, the evolution of gas permeability strictly follows the water content relationship between the samples. Therefore, the water content plays a predominant role at this stage. In addition, the microscopic image (after permeability tests) shows that the bentonite sample possesses a dense pore structure at high water content, that graininess is strong at low water content, and that a large number of pores exist. Regarding the effect of the gas pressure, it is found that the effect of gas pressure on the gas permeability is more evident at low water content. Gas does not adhere to the pore walls as liquid does, and the slippage of gases along the pore walls gives rise to an apparent dependence of permeability on pressure.